Page 15 - Spring 2006

open the valve...ultimately he succeeded, by seizing the cord with his teeth and dipping his head two or three times, until the balloon took a decided turn downward.” They probably reached at least 30,000 feet on this near-fatal flight.
Less hazardous but even more intriguing were the results from the rare night-time flights. On October 2, 1865, “When the sun had set for nearly three-quarters of an hour...the bal- loon left Woolwich Arsenal...the temperature at the time being 56° [F]...Within three or four minutes a height of 900 feet was reached...the temperature was 57° and increasing; on reaching 1,200 feet high it had increased to 58.9°. We then descended to 900 feet, and the temperature decreased to 57.8°; on beginning to ascend again the temperature increased to 59.6° at 1,900 feet high...in the several subse- quent ascents and descents the temperature increased with elevation, and decreased on approaching the earth...This result was remarkable indeed.”
While it was easy for these investigators to believe that the temperature would drop with altitude, they were aston- ished that, in some circumstances, the temperature increased with altitude. The “normal” drop in temperature produced upward refraction of sound and reduced audibility of sounds near the ground. The night-time temperature “inversion” would produce downward refraction of sound and, conse- quently, better audibility of distant sounds at night!
Reynolds wanted to prove that temperature gradients could refract sound but he found the experiments frustrat- ing. The effects produced by winds are often larger than the effects produced by temperature gradients: “...Mr. Glaisher’s ballon ascents in 1862...found that when cloudy the mean rate of diminution for the first 300 feet was 0.5° [F] for each 100 feet, and that when clear it was 1°...this rate of refraction is very small compared with that caused even by a very mod- erate wind... This renders the experiment very difficult to carry out...”
But the idea was provocative and it was worth trying to find conditions sufficiently calm. “In the hope of improving the conditions of the experiments, I accepted the invitation of my friend Major Hare, of Docking in West Norfolk, to accompany him in his yacht the ‘Feronia’ during a cruise on the east coast, taking rockets [as sources of sound] with me. Here I spend three weeks without having a single calm day.”
Success often follows persistence and Reynolds finally found a day with little wind and a daytime temperature inver- sion. He observed excellent sound transmission: “With regard to the cause of the exceptional distances over which we heard the sounds on the 19th of August, 1874...All the morning I had been watching the distant objects to see whether they were lifted or depressed by the refraction of light. They loomed to a remarkable degree that showed that the upward variation of temperature was the reverse of what I wanted...The looming of the distant objects showed that the air was colder below than above. This would tend to bring the sound down and intensify it at the surface of the water— in fact convert the sea into a whispering-gallery.”
Reynolds recognized that temperature gradients would refract both light and sound. He had hoped to find a calm day with a normal decrease in temperature with altitude and
Fig. 8. A temperature increase (an “inversion”) with altitude often occurs at night and this causes sound to be refracted downward, which enhances the sounds for an observer on the ground. On an expedition to Venezuela in 1899, Baron von Humboldt observed much better sound transmission from a waterfall on the Orinoco River at night than during the day. Such early observations revealed fea- tures of the surface layers of the atmosphere but temperature inversions are also a permanent feature of the stratosphere and account, in part, for long-range trans- mission of sound to hundreds of kilometers.
reduced audibility of sound but the enhanced sound trans- mission with a temperature inversion was just as convincing. The experiments of Reynolds and others eventually led to acceptance of refraction as a principal agent in producing wide variations in audibility of distant sounds. Reynolds acknowledged that there could be other effects including Tyndall’s flocculence: “With respect to the stoppage of the sound by the heterogeneity of the atmosphere...it seems to me that it must exist, but that it must at all times be confined to a very small distance above the earth’s surface and be over land.” Unfortunately, Reynolds’ explanation for the weakness of this form of scattering was flawed: “That, as a rule, there are no streams of heated air ascending to any considerable height over land, is definitely proved [!] by the fact that the light smoke from burning weeds never, or very seldom, attains an elevation of any thing like 100 feet.” Rising cur- rents of air can, in fact, reach tens of thousands of feet above the Earth’s surface; a fact exploited by soaring birds and glid-
er pilots.
Science rarely takes the shortest path from problem to
solution. When several effects operate simultaneously, we like to think that we can isolate the major effect, then remove it through a clever experiment and study the next most important effect. In reality, it may not even be obvious what the major effect is. For a century and a half after Derham’s paper, scattering by suspended particles (water, snow, hail) was taken to be the primary cause of the peculi- arities of sound transmission. Observational evidence did not support this, however, so Humboldt, Tyndall and others postulated that inhomogeneities (flocculence) from mixing processes caused excessive scattering (or absorption) of sound. Plausible, perhaps, but for audible frequencies in the atmosphere, refraction normally dominates; progress in understanding sound propagation was slow until this dom- inant mechanism was uncovered.
As soon as refraction of sound had reconciled the pecu- liarities of sound transmission over distances of tens of meters to kilometers, other, more bizarre observations— zones of silence interspersed between zones of audibility, for
Refraction of Sound in the Atmosphere 13